The macula is a small area in the retina of the eye, approximately 3 to 5 millimeters in size, adjacent to the optic nerve. It is the most sensitive area of the retina and contains the fovea, a depressed region that allows for high visual acuity and contains a dense concentration of cones, the photoreceptors that are responsible for color vision.
Macular degeneration is a term that refers to a number of different diseases characterized by degenerative changes in the macula, all of which lead to a loss of central vision. Age-related macular degeneration (ARMD) is the most common cause of functional blindness in developed countries for those over 50 years of age (Seddon, J M. Epidemiology of age-related macular degeneration. In: Ogden, T E, et al., eds. Ryan S J, ed-in-chief. Retina Vol II. 3rd ed. St. Louis, Mo.: Mosby; 2001:1039-50). The disease is characterized by progressive degeneration of the retina, retinal pigment epithelium (RPE), and underlying choroid (the highly vascular tissue that lies beneath the RPE, between the retina and the sclera). The retinal pigment epithelial layer is believed to be crucial for photoreceptor health. Cells in this layer recycle visual pigment (rhodopsin), phagocytose photoreceptor tips daily as part of rod and cone regeneration, and transport fluid across the membrane to the choroid, which is believed to help prevent detachment of the neural retina. Central vision deteriorates when cells in the RPE cease to function properly, which can lead to photoreceptor degeneration.
Despite extensive investigation, the pathogenesis of ARMD remains unclear, and the etiology of the molecular events that occur is not well understood. A variety of factors including oxidative stress, inflammation with a possible autoimmune component, genetic background (e.g., mutations), and environmental or behavioral features such as smoking and diet may contribute to the pathogenesis of ARMD in ways that are as yet poorly understood. Regardless of the underlying etiology, a clinical hallmark of ARMD is the appearance of drusen, localized deposits of lipoproteinaceous material that accumulate in the space between the RPE and Bruch's membrane, which separates the RPE from the choroidal vessels (choriocapillaris). Drusen are typically the earliest clinical finding in ARMD, and the existence, location, and number of drusen are used in classifying the disease into stages and for monitoring its progression (Ambati, J., et al., Surv. Ophthalmol., 48(3): 257-293, 2003; “Preferred Practice Pattern: Age-Related Macular Degeneration”, American Academy of Ophthalmology, 2003). Drusen are typically the earliest clinical finding in ARMD.
ARMD has been classified into both “dry” and “wet” (exudative, or neovascular) forms. Dry ARMD is much more common than wet ARMD, but the dry form can progress to the wet form, and the two occur simultaneously in a significant number of cases. Dry ARMD is typically characterized by progressive apoptosis of cells in the RPE layer, overlying photoreceptor cells, and frequently also the underlying cells in the choroidal capillary layer. Confluent areas (typically at least 175 μm in minimum diameter) of RPE cell death accompanied by overlying photoreceptor atrophy are referred to as geographic atrophy. Patients with this form of ARMD experience a slow and progressive deterioration in central vision.
Wet ARMD is characterized by bleeding and/or leakage of fluid from abnormal vessels that have grown from the choroidal vessels (choriocapillaris) beneath the RPE and the macula, which can be responsible for sudden and disabling loss of vision. It has been estimated that much of the vision loss that patients experience is due to such choroidal neovascularization (CNV) and its secondary complications. A subtype of neovascular ARMD in which angiomatous proliferation originates from the retina and extends posteriorly into the subretinal space, eventually communicating in some cases with choroidal new vessels has been identified (Yannuzzi, L. A., et al., Retina, 21(5):416-34, 2001). This form of neovascular ARMD, termed retinal angiomatous proliferation (RAP) can be particularly severe. The existence of macular drusen is a strong risk factor for the development of both wet and dry forms of ARMD (Ambati, J., et al., supra).
Treatment options for ARMD are limited, and none are fully effective (Ambati, J., et al., Surv. Ophthalmol., 48(3): 257-293, 2003, and references therein). Thus there is a need in the art for new approaches to the treatment of ARMD and also of other diseases and conditions of the eye characterized by macular degeneration, choroidal neovascularization, retinal neovascularization, retinal angiomatous proliferation, and/or blood vessel leakage. Such diseases and conditions include, but are not limited to, diabetic retinopathy and retinopathy of prematurity.